1. Field of the Invention
The invention generally relates to a system for remotely repairing nuclear fuel rod assemblies having damaged fuel rods or damaged skeletons. The system is capable of producing usable fuel rod assemblies by either replacing damaged fuel rods in an assembly having an intact skeleton, or by moving the usable fuel rods from an assembly having a damaged skeleton into an undamaged skeleton.
2. Description of the Prior Art
Tools for repairing damaged fuel rod assemblies are known in the prior art. Such tools may be used to either reconstitute a fuel rod assembly having damaged fuel rods, or to reassemble an assembly having a damaged support skeleton. In reconstitutiontype repairs, only the defective fuel rods from the undamaged support skeleton are pulled and replaced with new fuel rods. In reassembly-type repairs, all of the undamaged fuel rods are removed from the damaged support skeleton and inserted into a new, undamaged skeleton. In order to implement these two types of repair operations, manual tools have been developed for individually gripping, lifting, lowering and ungripping specific fuel rods from one support skeleton to another.
Unfortunately, the use of such prior art manual tools to implement the reconstitution or reassembly repair operations is not without shortcomings. However, before these shortcomings may be fully appreciated, some brief background as to the structure, operation, and environment of such fuel rod assemblies is necessary.
Nuclear fuel rod assemblies generally comprise 200 to 290 fuel rods mounted in a square array within a support skeleton. The support skeleton in turn is formed from bottom and top nozzle assemblies which are connected to one another by 24 uniformly arrayed thimble rods. The bottom and top nozzles are about 8 to 9 inches square, and the thimble rods are about 13 feet long, so that the overall shape of the fuel assembly is that of an elongated, rectangular prism (see FIG. 17). The fuel rods themselves are about 12 feet long. In order to equidistantly space the long and relatively flimsy fuel rods within the support skeleton, the skeleton includes approximately seven grids, each of which has a square array of apertures for receiving and spacing the fuel rods. The grids are usually sheet-metal structures fabricated from a heat-treated, high strength stainless steel in an "egg crate" type of design which lends compressive strength to the grids with a minimum of weight. In operation, an array of nuclear fuel rod assemblies is placed in the reactor core, and a jet of pressurzied water is guided through the bottom nozzles thereof in order to uniformly absorb the heat generated by the rods. In nuclear reactors of the type designed by Westinghouse Electric Corporation, the assignee of the present invention, the velocity of the pressurized water forced through the bottom nozzles of the fuel structures is on the order of 15 ft./sec.
In some nuclear cores, this 15-ft./sec. flow of water has created pressure differentials which in turn have resulted in side-currents which flow laterally through the fuel rod assemblies disposed in the core. These side currents sometimes produce vibrations in the fuel rods which can eventually weaken and break the rods through a fretting action.
In order to repair such damaged fuel rod assemblies, the damaged assembly is typically lowered into the cask-loading area (or shaft) of the spent fuel area of the nuclear plant. The cask-loading shaft is approximately 40 feet long, and filled with water in order to shield workers (who typically stand on a deck located over the shaft) from radiation. In reconstituting or reassembling such damaged fuel rod assemblies, the workers on the deck over the cask-loading shafts use elongated hand tools capable of gripping and withdrawing a single rod out of the fuel assembly after the top nozzle assembly has been cut and removed therefrom. Small television cameras are often mounted on these tools so that the workers may visually position them over a particular fuel rod. While such tools are capable of gripping, lifting, lowering and ungripping either damaged or undamaged fuel rods within a support skeleton, they are also long and flimsy, and hence slow and cumbersome to use. Additionally, if a worker attempts to pull a warped or bent fuel rod through the grids of a support skeleton, he has no positive way of knowing whether or not he is exerting an excessive tensile force upon the fuel rod. If too much tensile force is applied to the rod, it can break and contaminate the water in the cask-loading shaft with pellets of radioactive uranium oxide. Further, while the water in the cask-loading shaft does afford an effective shield for the majority of radiation emanating from the fuel rod assembly being repaired, workers positioned on the deck still receive some small amount of potentially hazardous radiation.
Clearly, there is a need for a system for automatically and remotely reconstituting or reassembling damaged fuel rod assemblies which is fast, efficient and safe. Ideally, such a system should include some sort of means for preventing an excessive amount of tensile force from being applied onto damaged fuel rods in order to minimize the probability that such rods will break and spill their respective radioactive contents. Finally, such a system should be completely remotely operable in order to insulate system operators from any amount of potentially harmful radiation.